Structure of N-linked oligosaccharides attached to chlorovirus PBCV-1 major capsid protein reveals unusual class of complex N-glycans

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 110; no. 34; pp. 13956 - 13960
• Main Authors: De Castro, Cristina, Molinaro, Antonio, Piacente, Francesco, Gurnon, James R., Sturiale, Luisa, Palmigiano, Angelo, Lanzetta, Rosa, Parrilli, Michelangelo, Garozzo, Domenico, Tonetti, Michela G., Van Etten, James L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 20.08.2013; National Acad Sciences
• Subjects: Biological Sciences; Capsid proteins; Capsid Proteins - chemistry; Capsid Proteins - isolation & purification; Capsid Proteins - metabolism; Chlorella - virology; Cytoplasm; DNA Ligases - chemistry; DNA Ligases - metabolism; Gas Chromatography-Mass Spectrometry; Glucose; Glycopeptides; Glycoproteins; Glycosylation; Magnetic Resonance Spectroscopy; Monosaccharides; Monosaccharides - chemistry; Oligosaccharides; Oligosaccharides - chemistry; Oligosaccharides - metabolism; Paramecium - microbiology; Peptides; Polysaccharides; Proteins; Protons; Spectral correlation; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; Spectroscopy; Viral Proteins - chemistry; Viral Proteins - metabolism; Viruses; glycopeptide; virus-encoded glycosylation; glucose/asparagine linkage; cytoplasmic glycosylation; glycobiology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1313005110

The major capsid protein Vp54 from the prototype chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1) contains four Asn-linked glycans. The structure of the four N-linked oligosaccharides and the type of substitution at each glycosylation site was determined by chemical, spectroscopic, and spectrometric analyses. Vp54 glycosylation is unusual in many ways, including: (i) unlike most viruses, PBCV-1 encodes most, if not all, of the machinery to glycosylate its major capsid protein; (ii) the glycans are attached to the protein by a β-glucose linkage; (iii) the Asn-linked glycans are not located in a typical N-X-(T/S) consensus site; and (iv) the process probably occurs in the cytoplasm. The four glycoforms share a common core structure, and the differences are related to the nonstoichiometric presence of two monosaccharides. The most abundant glycoform consists of nine neutral monosaccharide residues, organized in a highly branched fashion. Among the most distinctive features of the glycoforms are (i) a dimethylated rhamnose as the capping residue of the main chain, (ii) a hyperbranched fucose unit, and (iii) two rhamnose residues with opposite absolute configurations. These glycoforms differ from what has been reported so far in the three domains of life. Considering that chloroviruses and other members of the family Phycodnaviridae may have a long evolutionary history, we suggest that the chlorovirus glycosylation pathway is ancient, possibly existing before the development of the endoplasmic reticulum and Golgi pathway, and involves still unexplored mechanisms.